U.S. patent application number 12/309427 was filed with the patent office on 2010-02-25 for method for the early detection of damage to a capacitive sensor, and capacitive sensor featuring a diagnostic function.
Invention is credited to Ulrich Demisch, Juergen Hall, Markus Langenbacher.
Application Number | 20100045308 12/309427 |
Document ID | / |
Family ID | 37846237 |
Filed Date | 2010-02-25 |
United States Patent
Application |
20100045308 |
Kind Code |
A1 |
Hall; Juergen ; et
al. |
February 25, 2010 |
METHOD FOR THE EARLY DETECTION OF DAMAGE TO A CAPACITIVE SENSOR,
AND CAPACITIVE SENSOR FEATURING A DIAGNOSTIC FUNCTION
Abstract
A system for the early detection of damage to and/or soiling of
a capacitive sensor includes determining a degree by which the
sensor is damaged and/or soiled by measuring a physical property of
an electrode, e.g. the resistance of the electrode, or optically by
means of the coefficient of reflection of the electrode surface of
at least one electrode of the capacitive sensor. A capacitive
sensor, according to the system described herein, may include
components for provide the diagnostic function.
Inventors: |
Hall; Juergen; (Rotenbach,
DE) ; Langenbacher; Markus; (Schluchsee, DE) ;
Demisch; Ulrich; (Freiburg, DE) |
Correspondence
Address: |
MUIRHEAD AND SATURNELLI, LLC
200 FRIBERG PARKWAY, SUITE 1001
WESTBOROUGH
MA
01581
US
|
Family ID: |
37846237 |
Appl. No.: |
12/309427 |
Filed: |
July 21, 2006 |
PCT Filed: |
July 21, 2006 |
PCT NO: |
PCT/EP2006/007185 |
371 Date: |
October 2, 2009 |
Current U.S.
Class: |
324/658 ;
324/693; 356/237.1 |
Current CPC
Class: |
G01N 27/226 20130101;
G01N 33/007 20130101; G01N 27/4163 20130101; G01D 3/08
20130101 |
Class at
Publication: |
324/658 ;
324/693; 356/237.1 |
International
Class: |
G01N 27/22 20060101
G01N027/22; G01R 27/26 20060101 G01R027/26; G01N 21/00 20060101
G01N021/00 |
Claims
1. A method for early detection of damage to a capacitive sensor
comprising: determining an extent of the damage to the capacitive
sensor is by measuring a physical property of at least one
electrode of the capacitive sensor.
2. The method as recited in claim 1, wherein the extent of the
damage to the capacitive sensor is determined by measuring an ohmic
resistance across the at least one electrode.
3. The method as recited in claim 2, wherein the ohmic resistance
between a first terminal and a second terminal of the at least one
electrode is measured.
4. The method as recited in claim 2, wherein an impedance of a coil
situated in proximity of the at least one electrode, which is a
function of the ohmic resistance of the at least one electrode, is
measured.
5. The method as recited in claim 2, wherein an impedance of a
series circuit formed by a first auxiliary capacitor, an ohmic
resistor formed by the at least one electrode, and a second
auxiliary capacitor, which is a function of the ohmic resistance of
the at least one electrode, is measured.
6. The method as recited in claim 2, wherein an ohmic resistance of
at least one electrode is compared to a certain reference value,
and a comparison result is signaled by a certain logic level on a
signal line.
7. The method as recited in claim 1, wherein a reflection
coefficient of a surface of the at least one electrode is measured
and a measurement result is used for evaluating the damage to the
capacitive sensor.
8. The method as recited in claim 7, wherein the surface of the at
least one electrode is used as a reflector of a reflex
optocoupler.
9. The method as recited in claim 7, wherein light is injected into
a dielectric which is adjacent to the at least one electrode via a
first optic fiber, so that the light may propagate in the
dielectric, and the light is supplied from the dielectric to a
photodetector via a second optic fiber, the light being reflected
at least once on the surface of the at least one electrode on the
path from the first optical fiber to the second optical fiber
through the dielectric.
10. The method as recited in claim 9, wherein an intensity of the
reflected light is used for evaluating the damage to the capacitive
sensor.
11. The method as recited in claim 1, wherein the physical property
is measured continuously and a warning signal is triggered as soon
as a measured value for the physical property exceeds a reference
value.
12. The method as recited in claim 1, wherein the physical property
is measured regularly in certain time intervals and a variation
over time of measured values is analyzed for predicting a life
expectancy of the capacitive sensor.
13. A capacitive sensor, comprising: a first electrode; a second
electrode; a dielectric that separates the first electrode and the
second electrode, and a physical property measuring device that
measures a physical property of at least one of the electrodes.
14. The capacitive sensor as recited in claim 13, further
comprising: a resistance measuring device that measures an ohmic
resistance of the first electrode.
15. The capacitive sensor as recited in claim 14, wherein the first
electrode has a first terminal and a second terminal and the
resistance measuring device measures the ohmic resistance of the
first electrode between the two terminals.
16. The capacitive sensor as recited in claim 14, further
comprising: a coil situated in proximity of the first electrode,
the coil inducing eddy currents in the first electrode which are a
function of the ohmic resistance of the first electrode.
17. The capacitive sensor as recited in claim 14, further
comprising: a third electrode and a fourth electrode, wherein the
third electrode and the first electrode form a first auxiliary
capacitor, wherein the fourth electrode and the first electrode
form a second auxiliary capacitor, and wherein a resistor formed by
the first electrode, the first auxiliary capacitor, and the second
auxiliary capacitor form a series circuit.
18. The capacitive sensor as recited in claim 14, wherein an
elongated slot-shaped recess is provided in the first
electrode.
19. The capacitive sensor as recited in claim 14, wherein a
plurality of linearly arranged recesses are provided in the first
electrode.
20. The capacitive sensor as recited in claim 13, further
comprising: a light source and a photodetector, the light source,
the photodetector, and the first electrode forming a reflex
optocoupler which is used as a reflector for the first
electrode.
21. The capacitive sensor as recited in claim 13, further
comprising: a light source and a photodetector, wherein the
photodetector is connected to the dielectric adjacent to the first
electrode via a second optical fiber, wherein the light source is
connected to the dielectric via a first optical fiber, and wherein
light supplied to the dielectric through the first optical fiber is
reflected multiple times on a surface of the first electrode prior
to entering the second optical fiber.
22. The method according to claim 1, wherein the extent of damage
to the capacitive sensor includes an extent of soiling of the
capacitive sensor.
Description
[0001] The present invention relates to a method for early
detection of damage to a capacitive sensor and to a capacitive
sensor which is designed for early detection and signaling of
possible damage to the electrodes.
[0002] Capacitive sensors for determining dielectric properties of
gases and liquids are widely being used in measurement and process
technology. They are used in a plurality of industrial processes in
which they are exposed to the effect of aggressive and corrosive
substances. An incipient damage to the sensor, mostly a corrosion
of the sensor electrodes, is initially not noticed by the user, and
the damage often does not become apparent until total failure of
the sensor occurs. The result is a shutdown of the process for
several hours or longer until the defective device is replaced.
Such a total failure often also means substantial financial loss,
so that the user only has the option to frequently recalibrate the
sensor and replace it preventively in regular intervals, which is
also an unsatisfactory approach.
[0003] FIG. 1 shows a capacitive sensor according to the related
art. A first electrode 10 having a terminal 11 is situated on a
substrate 1. Above it, there is a second electrode 20 having a
terminal 21 which is separated from first electrode 10 by a
dielectric 30. A capacitance, which is a function of an electrical
property of the dielectric or other parameters affecting the
electrical properties of the dielectric, is measurable between
first terminal 11 of first electrode 10 and terminal 21 of second
electrode 20. If the sensor is exposed to aggressive substances
which are continuously damaging the electrodes (10, 20), there is
the risk of an unpredictable failure of the sensor with the
above-described consequences.
[0004] The present invention is based on the object of providing a
method for continuous monitoring of a capacitive sensor and for
early detection of damage in capacitive sensors, as well as a
sensor specially suited therefor.
[0005] This object is achieved by a method as recited in Patent
Claim 1 and a capacitive sensor as recited in Patent Claim 13.
Advantageous embodiments and refinements are the subject matter of
the subclaims.
[0006] The present invention is based on the recognition that the
extent of the damage to a capacitive sensor is directly derived
from a physical property or a physical parameter of a sensor
electrode. Such a physical property may be, for example, an
electrical property such as the ohmic resistance of a sensor
electrode, but also an optical property such as the reflection
coefficient of the electrode surface.
[0007] Measuring these physical properties is easily possible
without complicated adaptation of the sensor system proper. The
method according to the present invention provides for measuring
the physical property continuously or regularly in defined time
intervals during the operation of the sensor. If the measured value
differs excessively from a reference value (for example, the
measured value for an undamaged sensor), a warning signal is output
and the sensor should be replaced or recalibrated.
[0008] A typical capacitive sensor system includes a first
electrode having a first terminal and a second electrode having a
second terminal, the electrodes being separated by a dielectric.
The capacitance of the sensor system is measured via the two
terminals, from which in turn the desired electrical property of
the dielectric or parameters affecting the electrical properties of
the dielectric may be determined. Such a parameter may be, for
example, the moisture content in the dielectric. The use in
capacitive sensors for determining the quality of mineral oils or
edible oils or liquid edible fats is also possible.
[0009] In one specific embodiment of the present invention, a
second terminal is provided for the first electrode, so that in
addition to the sensor capacitance between the two electrodes, the
ohmic resistance of the first electrode between the first terminal
and the second terminal may also be measured. With increasing
damage to the sensor electrodes, the ohmic resistance of the first
sensor electrode will also increase, so that, depending on the
ohmic resistance, it may be decided whether a sensor is to be
recalibrated or replaced before total failure occurs.
[0010] To magnify the effect of the increase in resistance with
increasing damage to the sensor electrode, one or more slot-shaped
recesses may be provided in the electrode between the first
terminal and the other terminal of the first electrode, so that
thin webs are formed between the slot-shaped recesses or between a
slot-shaped recess and the edge of the electrode, which serve as
predetermined breaking points. For example, if four thin webs
through which a measuring current may flow are formed between the
first terminal and the other terminal of the first electrode, and
only one of these points breaks due to corrosion, the resistance of
the overall system between the first terminal and the other
terminal is increased by one-third. If three of the four webs
break, the resistance will be four times that of the undamaged
system. Furthermore, in the system illustrated in FIG. 3, the
direction of the current flow is rotated essentially by 90.degree.
with respect to the system illustrated in FIG. 2, i.e., the current
flows in the vertical direction, along the slot-shaped recess,
which in turn increases the length of the current path and thus the
total resistance. This results in advantages in analyzing the
measuring signal (for example, higher signal levels, etc.).
[0011] The resistance of the first electrode may, however, also be
measured in a contactless manner. In another specific embodiment of
the present invention, a coil is situated in the immediate
proximity of the first electrode, so that if an alternating signal
is supplied to the coil, an eddy current is induced in the first
electrode. The eddy current losses in the electrode and thus the
impedance of the coil are clearly a function of the ohmic
resistance of the first electrode, so that the ohmic resistance of
the electrode and thus the damage to the capacitive sensor may also
be inferred from the impedance of the coil.
[0012] In another, contactless specific embodiment, a third and a
fourth electrode are situated in such a way that the third
electrode and the first electrode, as well as the fourth electrode
and the first electrode each form an auxiliary capacitor, so that a
series circuit of a first auxiliary capacitor and an ohmic resistor
and a second auxiliary capacitor is obtained, the ohmic resistor
being formed, as previously, by the first electrode. The total
impedance of this series circuit is also a function of the ohmic
resistance of the first electrode; however, in addition, the
capacitance values of the first and second auxiliary capacitors
also change as a function of the damage to the sensor. Also in this
case, the damage to the sensor may be inferred from the impedance
of the series circuit, an additional effect, namely the change in
the capacitance value of the auxiliary capacitors, contributing to
the overall measured effect.
[0013] Another option for evaluating the damage to the sensor is by
measuring an optical property such as, for example, the reflection
coefficient of the first electrode. In such a specific embodiment,
the first electrode of the capacitive sensor forms the reflector in
a reflex optocoupler. A light source and a photodetector are
situated in such a way that the light emitted by the light source
is reflected on the surface of the first electrode prior to being
received by the photodetector. In the event of increasing damage to
the sensor, for example, by corrosion of the electrode, the
reflection properties of the electrode are modified, so that the
extent of the damage to the electrode may be determined from the
intensity of the received light. Corrosion of the electrode affects
both the scattering properties and the absorption properties of the
electrode. The measuring effect is therefore such that, on the one
hand, the absorption coefficient of the surface is modified and, on
the other hand, the scattering properties are modified, so that the
proportion of the light scattered toward the photodetector is
modified. The intensity of the light received by the photodetector
is a measure of the reflection coefficient of the electrode surface
and thus a measure of the damage to the sensor. When the sensor is
optically monitored, a mere soiling of the sensor electrodes may
also be detected in addition to actual damage.
[0014] In a specific embodiment, light is injected into the
dielectric between the two electrodes of the capacitive sensor via
an optical fiber, so that the light is reflected multiple times
back and forth between the two electrodes. The light reflected
multiple times is extracted again via a second optical fiber and
supplied to a photodetector. Also in this case, the intensity of
the reflected light is a measure of the damage to the sensor. The
principle is the same as described previously. The measuring effect
is a modification in the absorption and scattering properties of
the electrode surfaces.
[0015] The method according to the present invention allows a
preferably continuous monitoring of the capacitive sensor by
continuously measuring a suitable physical property (resistance,
reflection coefficient, scattering properties, etc.) of an
electrode. Of course, periodically or aperiodically repeated
individual measurements are also possible. As soon as a measured
value for the physical property exceeds a reference value, a
warning signal, for example, may be triggered, whereupon the
replacement of the corresponding sensor may be initiated. In
addition, a prediction may be made from the variation of the
measuring signal over time (for example, the variation of the
measured electrode impedance) of how long the sensor may still be
used under conditions that remain constant.
[0016] The present invention is elucidated below in greater detail
with reference to exemplary embodiments illustrated in the
figures.
[0017] FIG. 1 shows a capacitive sensor according to the related
art, without a diagnostic function;
[0018] FIG. 2 shows a capacitive sensor having a first electrode
and a second electrode, the first electrode having a first terminal
and a second terminal, between which the ohmic resistance of the
first electrode may be measured;
[0019] FIG. 3 shows a capacitive sensor such as in FIG. 2, in which
additionally a slot-shaped recess is provided in the first
electrode;
[0020] FIG. 4 shows a capacitive sensor such as in FIG. 2, in which
a plurality of slot-shaped recesses is provided in the first
electrode;
[0021] FIG. 5 shows a capacitive sensor having an additional third
electrode and an additional fourth electrode, the third electrode,
the first electrode, and the fourth electrode forming a series
circuit of a first capacitor, an ohmic resistor, and a second
capacitor;
[0022] FIG. 6 shows a capacitive sensor such as in FIG. 1, in which
a coil is additionally provided for determining the ohmic
resistance of the first electrode indirectly via the impedance of
the coil;
[0023] FIG. 7 shows a capacitive sensor in which the first
electrode forms the reflector of a reflex optocoupler;
[0024] FIG. 8 shows a capacitive sensor in which the light is
injected into the dielectric between the electrodes, where it is
reflected multiple times;
[0025] FIG. 9a shows the electrical equivalent circuit diagram for
the sensor systems of FIGS. 2 through 4;
[0026] FIG. 9b shows the electrical equivalent circuit diagram for
the sensor system of FIG. 5, and
[0027] FIG. 9c shows the electrical equivalent circuit diagram for
the sensor system of FIG. 6.
[0028] In the figures, the same reference numerals identify the
same elements.
[0029] FIG. 1 schematically shows a capacitive sensor without a
diagnostic function according to the related art. As described in
the introduction, this sensor system includes a first electrode 10
having a first terminal 11 and a second electrode 20 having a
second terminal 21. Capacitance C.sub.M is measurable between first
terminal 11 of first electrode 10 and terminal 21 of second
electrode 20.
[0030] FIG. 2 shows a capacitive sensor according to the present
invention having a diagnostic function. The sensor includes a first
electrode 10 having a first terminal 11 and a second terminal 12,
and a second electrode 20 having a terminal 21. The desired
capacitance C.sub.M may be measured between first terminal 11 of
first electrode 10 and terminal 21 of second electrode 20, as in
the capacitive sensor according to the related art. Between its
first terminal 11 and its second terminal 12, first electrode 10
represents an ohmic resistance R.sub.E, which is measured for
diagnostic purposes. This ohmic resistance R.sub.E increases with
increasing damage to first electrode 10. The measured value for
R.sub.E is therefore used for evaluating the extent of the damage
to the sensor.
[0031] To magnify this effect, a slot-shaped recess 13 may be
provided in the first electrode, so that a narrow web 14 is formed
between first terminal 11 and second terminal 12 of first electrode
10. Such a system is shown in FIG. 3. Due to the narrow web, the
current may flow basically only along slot-shaped recess 13,
whereby a longer current path and thus a measured resistance
R.sub.E that is easier to evaluate is achieved compared to the
system shown in FIG. 2.
[0032] In the specific embodiment shown in FIG. 4, a plurality of
slot-shaped recesses 13 is situated between first terminal 11 and
second terminal 12 of first electrode 10 next to each other along
two parallel lines, so that a plurality of narrow webs 14 is
created. These webs 14 are used almost as predetermined breaking
points which break in the event of increasing corrosion of first
electrode 10, whereby ohmic resistance R.sub.E between first
terminal 11 and second terminal 12 increases. This effect is
considerably stronger than the increase in resistance in a system
according to FIG. 2. The electrical equivalent circuit diagram of
the specific embodiments shown in FIGS. 2 through 4 is illustrated
in FIG. 9a.
[0033] In the specific embodiments illustrated in FIGS. 5 and 6 the
electrode resistance is measured in a contactless manner. In the
sensor illustrated in FIG. 5, a third electrode 18 and a fourth
electrode 19 are situated in such a way that third electrode 18 and
first electrode 10 form a first auxiliary capacitor C.sub.1, and
fourth electrode 19 and first electrode 10 form a second auxiliary
capacitor C.sub.2. First auxiliary capacitor C.sub.1 and second
auxiliary capacitor C.sub.2 are connected in series via ohmic
resistor R.sub.E formed by first electrode 10. The electrical
equivalent circuit diagram of this system is illustrated in FIG.
9b. In this specific embodiment of the present invention, impedance
Z.sub.1 of the series circuit is measured, which is clearly a
function of ohmic resistance R.sub.E of first electrode 10, but
also of the capacitances of auxiliary capacitors C.sub.1 and
C.sub.2. The extent of damage to the capacitive sensor may thus be
inferred from the measured impedance Z.sub.1. The measuring effect
is not only a change in ohmic resistance R.sub.E, but also a change
in the capacitances of auxiliary capacitors C.sub.1 and C.sub.2 due
to damage to the electrodes, whereby the overall effect is further
enhanced.
[0034] In this specific embodiment, sensor capacitor C.sub.M is
formed by a series circuit of a first sensor capacitor C.sub.M1 and
a second sensor capacitor C.sub.M2. First sensor capacitor C.sub.M1
is formed by electrodes 20A and 10; second sensor capacitor
C.sub.M2 is formed by electrodes 20B and 10. Due to the series
circuit, the equation
C.sub.M=(C.sub.M1C.sub.M2)/(C.sub.M1+C.sub.M2) applies; the
capacitance of the sensor capacitor is to be measured between the
terminals of electrodes 20A and 20B. One advantage of this
capacitive system is also that electrode 10 to be monitored does
not need to have any electrical connection.
[0035] A second contactless system is shown in FIG. 6. A coil 50 is
situated in the immediate proximity of first electrode 10 in such a
way that if an alternating signal is supplied to the coil, an eddy
current is induced in first electrode 10. On the one hand,
impedance Z.sub.2 of coil 50 is a function of the inductance of
coil 50; on the other hand, it is a function of the eddy current
losses which may be modeled in an equivalent circuit diagram (see
FIG. 9c) as series resistance R.sub.W to coil 50. This equivalent
circuit diagram is shown in FIG. 9c. Series resistance R.sub.W is
clearly proportional to resistance R.sub.E of first electrode 10.
Also in this case, ohmic resistance R.sub.E of first electrode 10
and thus the extent of the damage to the sensor may be inferred
from impedance Z.sub.2 of the coil. The electrical equivalent
circuit diagram of this system is illustrated in FIG. 9c.
[0036] In another specific embodiment of the present invention, the
reflection coefficient of first electrode 10 is used as a measure
of the damage to the sensor. The surface of first electrode 10 is
then used as a reflector in a reflex optocoupler. The system
includes a light source 40 and a photodetector 45, the light
emitted by light source 40 being reflected on the surface of first
electrode 10 prior to being received by photodetector 45. The
intensity of the light received by photodetector 45 is a measure of
the reflection coefficient of first electrode 10 and is used as a
measure of the damage to the capacitive sensor. Depending on the
application, when the sensor is optically monitored, a mere soiling
of the sensor electrodes may also be detected in addition to actual
damage.
[0037] The exemplary embodiment illustrated in FIG. 8 is based on
the same principle; in this embodiment the light of a light source
40 is injected into dielectric 30 between first electrode 10 and
second electrode 20 via a first optical fiber in such a way that it
is reflected multiple times back and forth between first electrode
10 and second electrode 20 prior to being extracted via a second
optical fiber from the dielectric and supplied to a photodetector
45. The intensity of the light received by the photodetector is a
measure of the reflection coefficient of the electrode surface and
thus a measure of the damage to the capacitive sensor.
[0038] FIG. 9a shows the electrical equivalent circuit diagram for
the specific embodiments of FIGS. 2 through 4. Sensor capacitor
C.sub.M formed by electrodes 10 and 20 and ohmic resistor R.sub.E
formed by first electrode 10 are shown. Resistor R.sub.E is
situated between terminals 11 and 12; sensor capacitor C.sub.M is
situated between terminals 11 and 21.
[0039] FIG. 9b shows the equivalent circuit diagram of the specific
embodiment of FIG. 5. Impedance Z.sub.1 of the system has a series
circuit made up of first auxiliary capacitor C.sub.1 formed by
electrodes 18 and 10, resistor R.sub.E formed by first electrode
10, and a second auxiliary capacitor formed by electrodes 10 and
19. The sensor capacitor itself is not illustrated in this
figure.
[0040] FIG. 9C shows the equivalent circuit diagram of the specific
embodiment of FIG. 6. The eddy current losses caused by coil 50 in
first electrode 10 work as losses in an ohmic series resistor
R.sub.W in series with coil 50. Impedance Z.sub.2 of the coil is
thus formed by a series circuit made up by an inductor and series
resistor R.sub.W, series resistance R.sub.W being proportional to
resistance R.sub.E of first electrode 10 in which eddy currents are
induced.
[0041] A statistical analysis of the variation of the measuring
signal over time (for example, the variation of the measured
electrode impedance or of the measured reflected light intensity)
allows a prediction to be made of how long the sensor may still be
used under conditions that remain constant.
[0042] For this purpose, the original value of the corresponding
physical parameter (i.e., electrode resistance, impedance,
reflection coefficient, etc.) of the sensor must be measured at the
end of the manufacturing process and saved in the sensor. During
operation, the corresponding physical parameter is measured
continuously or in certain time intervals and the measured values
are saved together with a time stamp (i.e., together with the point
in time of the measurement). From the saved measured values and the
corresponding time stamps, a regression analysis is performed,
i.e., a (for example, linear) trend is computed with the aid of a
(for example, linear) regression function. Either polynomial
functions or exponential functions may be used as regression
functions. With the aid of the regression function, a point in time
is extrapolated at which the corresponding physical parameter
exceeds the critical threshold value which indicates unacceptably
strong damage to the sensor. The user receives suitable information
about the predictably necessary point in time of replacement of the
sensor and may still plan a service date in a timely manner to
prevent further damage.
* * * * *